The present invention relates to a data-reproducing system for digital recording and reproducing devices using the PBML signal-processing technique, such as hard disk drives. More specifically, the present invention relates to a preamble pattern for initial acquisition in AGC and PLL processing in front end processing and to an initial acquisition procedure using the preamble pattern.
The conventional magnetic-recording channel is of the so-called longitudinal recording system type. According to this system, information is recorded in the form of plus magnetization and minus magnetization arranged parallel to the recording surface. Various pieces of information are recorded on magnetic disk media of the longitudinal recording system type in accordance with a format such as is shown in FIG. 14. The servo pattern is provided in a region to record a bit pattern for tracking and addressing, which is used to position the magnetic head. The magnetic head records and reproduces data based on the bit pattern. A gap is inserted after the servo pattern to cope with a variation of the rotation of the disk, and a data sector follows the gap. The data sector is a region where a certain pattern called a “preamble” is recorded repeatedly. By referring to this region, initial acquisition by an AGC circuit and a PLL circuit is carried out. Thereafter, information of a synchronization pattern (SYNC) is read, and the data is reproduced.
In the recording format of a hard disk drive, the preamble region is indispensable for reproducing data, but the provision of this preamble region is one of the factors reducing the data efficiency; accordingly, it is preferable to keep the preamble region as small as possible. A number of methods have been proposed from various viewpoints to control the amplitude and timing accurately, using as small a number of bits as possible.
In U.S. Pat. No. 5,258,933, there is disclosed a technique for making use of a zero-cross comparator to convert a signal reproduced from a preamble into binary codes. A zero-cross timing pulse is obtained by the zero-cross comparator, and this timing pulse is fed to a VCO to stop its oscillation for a certain time. Then the VCO resumes oscillation one half of the duration of one bit after the next zero-cross timing pulse. Thus, the phase shift between the reproduced signal and the VCO clock is reduced to almost zero to shorten the time necessary for phase acquisition.
U.S. Pat. No. 5,552,942 discloses another technique. According to this technique, the phase of a VCO clock is optimally controlled by calculating the mean square errors of expected values and actual values of samples of a known preamble pattern.
Japanese Patent Laid-Open No. 315517/1996 discloses a technique for expanding the capture range to ±T (T is the duration of one bit) as effective measures in case the SN (signal-to-noise) ratio of the preamble pattern goes down.
According to the conventional timing-controlling method, a timing gradient is calculated from the amplitude value X(k) at the k-th sampling point and the judged value r(k) of X(k) by using the following expression:ΔT(k)=−4(k−1)×X(k)+r(k)×X(k−1)and the timing of the VCO clock is renewed so as to converge the result to zero. The ideal capture range is ±T/2. However, if r(k) is judged erroneously due to noise, the timing gradient may exceed the capture range of ±T/2, elongating the time necessary for phase acquisition. To prevent it, r(k) is not judged at every sampling point, but is predicted by making use of the periodicity of the preamble pattern. If the preamble pattern is of a 4T cycle, r(k) has the repetition of (+1, +1, −1, −1); therefore, the timing gradient at every second sampling point is given by the expression below.ΔT(k)=X(k)−X(k−2)
Japanese Patent Laid-Open No. 21096/2000 discloses a technique to obtain a timing clock for sampling by using a preamble pattern which is not of a 4T cycle. This technique relates to the PR5 equalization system, which is suitable to magnetic-recording channels for reproducing data of a single-layer perpendicular-recording medium with a ring head. A timing clock for sampling is obtained by using a preamble pattern consisting of repetition of a magnetization pattern of “+1, +1, +1, −1, −1, −1” in PBML processing, wherein a single-layer perpendicular-recording medium and a ring head are combined with the PR5 system.
The development of a perpendicular-recording system has been accelerating recently, and there is a good possibility that most media will shift from the longitudinal recording system to the perpendicular-recording system because the latter is suitable to high-density recording.
Unlike the longitudinal recording channel which shows band-pass filter characteristics, the response of reproduced signals of a perpendicular-recording medium has a spectrum in a low-frequency area adjacent to DC, and it is known that the waveform responding to isolated transition of perpendicular magnetization can be well approximated by the hyperbolic tangent function tanh (x).
Due to improvement of the recording density and of the reproduction sensitivity by GMR heads and so on in recent magnetic systems, there is a tendency for the noise arising from recording media to become more salient than the noise arising from the circuits of systems. An example of the medium noise is jitter-like noise due to the variation of the transition point in magnetization. As the jitter-like noise becomes predominant over the system noise, which is white noise, it becomes difficult to improve the performance of hard disk drives of the PRML that make making use of a spectrum of lower frequency.
Desirable in a perpendicular magnetic recording system, wherein a perpendicular-recording two-layer medium and a single magnetic-pole head are combined, is the PR(1, 1) system, the PR(1, 2, 1) system, or the PRML system, which is based on the former two systems, and wherein equalization is effected according to the nature of the noise on the channel and data is demodulated by an ML, each system being capable of using the spectrum in the low-frequency area effectively. In this case, the provision of means for controlling timing and amplitude to sample a reproduced signal at proper timing and amplitude is indispensable.
The most simple method of accomplishing the above is as follows. A reproduced signal with a spectrum of lower frequency and a differentiated signal obtained by differentiating the reproduced signal are prepared. The timing for sampling is controlled and the gain is adjusted by applying a circuit of initial acquisition by a conventional 4T-cycle preamble pattern to the differentiated signal, and the reproduced signal with a spectrum of lower frequency is sampled with a phase-controlled clock. In the case of this method, it is necessary to provide means for switching between the reproduced signal with a spectrum of lower frequency and the differentiated signal, as well as a means for correcting the relative time difference between the reproduced signal with a spectrum of lower frequency and the differentiated signal.
If there is heavy jitter-like noise due to variation of the point of magnetization transition, the amplitude levels at sampling points fluctuate to cause large errors in computed timing and amplitude gradients. There are four sampling points in one cycle of a 4T-cycle preamble. If white noise is added to it, averaging can be effected four times a cycle, and hence the variance of timing gradients is reduced to 1/√4=½. On the other hand, because jitter-like noise is caused by the displacement of the point of magnetization transition, both the former two sampling points and the latter two sampling points are saliently correlated and tend to move at a level in the same direction; accordingly, averaging can be effected only two times a cycle in substance. Thus, if jitter-like noise is predominant over white noise, the errors of computed timing and amplitude gradients are difficult to reduce. If the loop gain is set high at an early stage of acquisition to reduce the time necessary for the acquisition in such a channel, the set values of the VCO and the VGA may be affected considerably when the loop gain is switched to another value. In this case, if the loop gain is switched to the former value, the acquisition does not finish in the remaining part of the preamble; therefore, the loop gain can not be set high in acquisition, which makes the acquiring time long, increases the number of bits to be allotted to the preamble pattern, and reduces the data efficiency.
In the case of the technique of zero-phase start disclosed in the U.S. Pat. No. 5,552,942, because the mean square error has to be calculated a number of times by changing the timing of sampling to find a timing which gives a minimum mean square error, a large number of bits have to be allotted to the preamble pattern.
In the case of the technique of extracting a clock for the correct timing of sampling by using a 6T-cycle preamble pattern disclosed in the Japanese Patent Laid-Open No. 2000-21096, the effects of jitter-like noise are not dealt with. Because signals after equalization by the PR5 system are used and a magnetic-recording channel with band-pass filter characteristics similar to the PR4's characteristics is assumed, it is difficult to reduce the number of bits to be allotted to the preamble.
The above two techniques deal with the extraction of the timing clock, but do not deal with the effects of jitter-like noise in amplitude adjustment.